1. Field of the Invention
The present invention relates to an electronic flash device having a boosting circuit and to a power supply circuit.
2. Description of Related Art
In a boosting circuit which charges a capacitor through a secondary winding of a boosting transformer by turning on and off a transistor connected to a primary winding of the boosting transformer by self-excited oscillation, the oscillation frequency becomes high in general when the voltage of the capacitor becomes higher, i.e., when a charging current to the capacitor from the secondary winding becomes smaller.
In such a condition, a current flowing at the primary winding of the transformer immediately before turning-off of the transistor inevitably arises in order for the transistor to shift to its off-state. However, the current flowing at the primary winding at that time does not contribute to a charging action on the capacitor and becomes useless energy. In the conventional boosting circuit, such useless energy is repeatedly generated at the above-stated high frequency of oscillation, thereby causing the great loss of energy.
The details of the conventional boosting circuit are as described below with reference to FIGS. 5, 6 and 7.
In FIG. 5, reference numeral 1 denotes a power supply battery, numeral 2 denotes a power supply switch. One end of the power supply switch 2 is connected to the plus terminal of the power supply battery 1 and the other end is connected to the emitter of a PNP transistor 5. A resistance 3 and a capacitor 4 each are connected between the base and the emitter of the PNP transistor 5. The PNP transistor 5 has its emitter connected to the plus terminal of the power supply battery 1, its collector connected to one end of a primary winding of a boosting transformer 7 and its base connected to one end of a feedback winding of the boosting transformer 7.
The primary winding of the boosting transformer 7 has its one end connected to the collector of the transistor 5 and the other end connected to the minus terminal of the power supply battery 1. The feedback winding of the boosting transformer 7 has its one end connected to the base of the transistor 5 and the other end connected to a resistance 6. The secondary winding of the boosting transformer 7 has its one end connected to the anode of a rectifying diode 8 and the other end connected to the base of the transistor 5. The rectifying diode 8 has its cathode connected to the anode of a main capacitor 9. The main capacitor 9 is arranged to supply a flashing energy to a xenon discharge tube 17. The main capacitor 9 has its anode connected to the cathode of the diode 8 and its cathode connected to the minus terminal of the power supply battery 1.
A resistance 10 has its one end connected to a flashing signal transmission circuit 18 and the other end connected to the gate of an SCR (silicon controlled rectifier) 14. A resistance 11 and a capacitor 12 are connected in parallel. The parallel circuit thus obtained is connected between the gate and the cathode of the SCR 14. A resistance 13 has its one end connected to the anode of the main capacitor 9 and the other end connected to the anode of the SCR 14.
A capacitor 15 for triggering has its one end connected to the anode of the SCR 14 and the other end connected to one end of the primary winding of a trigger transformer 16. The SCR 14 has its gate connected to one end of the resistance 10, its anode connected to one end of the capacitor 15 and its cathode connected to the cathode of the main capacitor 9. The trigger transformer 16 has a primary winding and a secondary winding. The primary winding of the trigger transformer 16 has its one end connected to one end of the capacitor 15 and the other end connected to the cathode of the SCR 14. The secondary winding of the trigger transformer 16 has its one end connected to the trigger electrode of the xenon discharge tube 17 and the other end connected to the cathode of the SCR 14.
The xenon discharge tube 17 is arranged to make a flash emission, and has its anode connected to the anode of the main capacitor 9 and its cathode connected to the cathode of the main capacitor 9. The flashing signal transmission circuit 18 is arranged to generate a high-level signal upon receipt of a flashing start signal from a camera (not shown).
The resistances 10, 11 and 13, the capacitors 12 and 15, the SCR 14, the trigger transformer 16, the xenon discharge tube 17 and the flashing signal transmission circuit 18 shown in FIG. 5 jointly constitute a known xenon tube flashing circuit. In this circuit, the xenon discharge tube 17 emits a flash light in response to the flashing start signal. However, the details of the operation of the xenon tube flashing circuit are omitted herein.
A voltage boosting action on the main capacitor 9 shown in FIG. 5 is next described as follows. When the power supply switch 2 is turned on, a base current of the transistor 5 which serves as a switching element flows through the feedback winding of the boosting transformer 7 and the resistance 6, and a collector current of the transistor 5 flows to the primary winding of the boosting transformer 7. Then, an oscillation circuit which is composed of the resistances 3 and 6, the capacitor 4, the transistor 5 and the boosting transformer 7 begins to perform self-excited oscillation.
Operation of the boosting circuit to be performed when the voltage of the main capacitor 9 is at a low level is next described with reference to FIG. 7. Referring to FIG. 7, when the base current of the transistor 5 flows at a point of time t4, an increase of the feedback current is fed back to the primary winding of the boosting transformer 7. As a result, the current flowing to the primary winding of the transformer 7, i.e., the collector current of the transistor 5, increases. The increase of the current flowing to the primary winding of the transformer 7 is fed back to the feedback winding thereof to increase the base current of the transistor 5, so that the transistor 5 is completely turned on.
With the transistor 5 turned on, a charging current flows to the main capacitor 9 through the diode 8 which is connected to the secondary winding of the boosting transformer 7. At this time, the collector current of the transistor 5 becomes a very large current between a point of time t4 and another point of time t5 as shown in FIG. 7. This is because the voltage of the main capacitor 9 is at a low level. Thus, the charging current flowing to the main capacitor 9 when the voltage of the main capacitor 9 is low is much larger than a charging current flowing to the main capacitor 9 when the voltage of the main capacitor 9 is high.
A current which is irrelevant to the supply of current to the secondary winding of the transformer 7 gradually increases accordingly as time elapses from the point of time t4. Then, this current causes the magnetic flux density of a magnetic substance of the transformer 7 to increase also gradually. When this current reaches a level Ip shown in FIG. 7, the magnetic flux density of the magnetic substance reaches its saturation area, so that the feedback action becomes no longer performed, with the result that the transistor 5 shifts to its off-state.
When the transistor 5 is in its on-state, the collector current of the transistor 5 is much larger than the current Ip, i.e., several to scores of times as much as the current Ip. Further, the transistor 5 shifts to its off-state at the point of time t5. The period between the points of time t4 and t5 is caused to vary by the voltage of the main capacitor 9, and is longer accordingly as the voltage of the main capacitor 9 is lower. When the transistor 5 is turned off at the point of time t5, energy stored in the boosting transformer 7 causes the output of the feedback winding to oscillate. With the output of the feedback winding oscillating, a reverse bias is applied for a predetermined period of time to the base of the transistor 5 and, after that, a forward bias is applied to cause a base current of the transistor 5 to flow. Then, the action from the point of time t4 is repeated, and the oscillation is caused to continue.
Generally, in the boosting transformer 7, a current which is irrelevant to the supply of current to the secondary winding is much smaller than the collector current of the transistor 5. Therefore, the loss of that current is very small and hence inconsequential.
Operation of the boosting circuit to be performed when the voltage of the main capacitor 9 is high is next described with reference to FIG. 6. When a base current of the transistor 5 begins to flow at a point of time t1 due to self-excited oscillation, the transistor 5 is turned on, as mentioned above with reference to FIG. 7. After that, the main capacitor 9 is charged through the secondary winding of the transformer 7 and the diode 8. However, since the voltage of the main capacitor 9 is high in this instance, the charging current is small and, therefore, the collector current of the transistor 5 is also small.
However, as mentioned above, a current which is irrelevant to the supply of current to the secondary winding of the boosting transformer 7 gradually increases. This current causes the magnetic flux density of the magnetic substance of the transformer 7 to gradually increase. Then, the magnetic flux density begins to become saturated at a point of time t2. Then, the collector current of the transistor 5, i.e., the current flowing to the primary winding of the boosting transformer 7, rapidly increases. When this current reaches a level Ip shown in FIG. 6, the magnetic flux density of the magnetic substance reaches its saturation area, so that the feedback action becomes no longer performed, with the result that the transistor 5 shifts to its off-state. The current Ip is larger than the current obtained between the points of time t1 and t2. Further, a period between the points of time t2 and t3 is a value which is not inconsequential as compared with the period between the points of time t1 and t2. The large loss of current thus takes place during the period between the points of time t2 and t3.
In the conventional boosting circuit described above, the switching transistor 5 on the side of the primary winding of the transformer 7 is turned on when the base current of the transistor 5 is caused to begin to flow at the point of time t1 shown in FIG. 6 by the self-excited oscillation. The main capacitor 9 is then charged through the secondary winding of the transformer 7. Since the voltage of the main capacitor 9 is high in this instance, the charging current is small and, therefore, the collector current of the transistor 5 is also small.
However, as mentioned above, the amount of a current which is irrelevant to the supply of current to the secondary winding of the transformer 7 gradually increases. This current causes the magnetic flux density of the magnetic substance of the transformer 7 to gradually increase. The magnetic flux density begins to become saturated from the point of time t2. Then, the collector current of the transistor 5, i.e., the current flowing to the primary winding of the transformer 7, rapidly increases. When the collector current of the transistor 5 reaches the level Ip, as shown in FIG. 6, to bring the magnetic flux density of the magnetic substance of the transformer 7 to its saturation area, the feedback action ceases to be performed, and the transistor 5 shifts to its off-state. The current Ip is larger than the current obtained between the points of time t1 and t2. Further, the period between the points of time t2 and t3 is a value which is not inconsequential as compared with the period between the points of time t1 and t2. The large loss of current thus takes place during the period between the points of time t2 and t3.
Further, in the conventional boosting circuit, when the voltage of the power supply battery is low (i.e., when the load is large), an oscillating action of the self-excited oscillation circuit is inhibited. Accordingly, when the voltage of the power supply battery becomes lower, the oscillating action is made to be performed intermittently, thereby causing the oscillation transformer to generate an uncomfortable oscillation sound.